This invention relates to a method of operating a hybrid electric vehicle to reduce tailpipe emissions and more particularly, to a method of operating a hybrid electric vehicle which utilizes the vehicle""s electric motor/generator to reduce emissions during cold-start and transient conditions.
Conventional vehicles having internal combustion engines utilize a three-way-catalyst (xe2x80x9cTWCxe2x80x9d) to reduce tailpipe emissions. Particularly, the TWC catalytically reduces nitrogen oxides (NOx) and oxidizes carbon monoxide (xe2x80x9cCOxe2x80x9d) and unburned hydrocarbons (xe2x80x9cHCxe2x80x9d) which are produced during the combustion process. The TWC has a very high conversion efficiency once the catalyst has xe2x80x9cwarmed upxe2x80x9d and the air-fuel ratio of the mixture is near its stoichiometric point. An example of the conversion efficiency of a typical catalytic converter over time is shown in graph 100 of FIG. 5.
In conventional vehicles, more than fifty percent (50%) of the HC and CO emissions are generated in the first sixty seconds of a standard emissions test cycle (e.g., the EPA75 test cycle), and more than twenty five percent (25%) of the NOx emissions are generated during that time. An example of the tailpipe emissions of a vehicle during a standard emissions test is shown in graph 110 of FIG. 6. The point in time at which the catalytic converter reaches a fifty percent (50%) efficiency is commonly referred to as its xe2x80x9clight-offxe2x80x9d time. Due to the relatively poor efficiency of the catalytic converter prior to xe2x80x9clight-offxe2x80x9d, recent efforts to reduce tailpipe emissions have concentrated on reducing the xe2x80x9clight-offxe2x80x9d time, thereby reducing the time during which the catalytic converter is least efficient. These prior efforts have also included concomitantly altering the air-fuel ratio and/or retarding the spark calibration of the engine.
These prior efforts have suffered from some drawbacks. Particularly, the difficulty in controlling the combustion stability of the engine and the operating load of the engine as it warms up severely limits these prior strategies. Moreover, although significant fractions of the emissions are produced during xe2x80x9ccold startxe2x80x9d type conditions (e.g., during the first sixty seconds of vehicle operation), periods when engine operating loads are changing rapidly or xe2x80x9ctransient eventsxe2x80x9d also produce a significant portion of the emissions, specifically NOx emissions (see e.g., FIG. 6). Hence, these methods which concentrate on cold-start type operating conditions do not adequately address or improve emissions during transient events once the vehicle has warmed up.
Hybrid electric vehicles have been designed and manufactured for the purpose of improving fuel economy and emissions. Particularly, hybrid electric vehicles utilize both an internal combustion engine and one or more electric motors to generate power and torque. The electric motor(s) within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering power and torque. While hybrid electric vehicles significantly reduce emissions, the foregoing emissions reducing strategies are not well-suited for use with hybrid electric vehicles. Particularly, the foregoing emissions reducing strategies do not maximize and/or utilize the flexibility of hybrid electric vehicles to utilize both an electric motor and an internal combustion engine to provide power and torque.
There is therefore a need for a method of operating a hybrid electric vehicle to reduce emissions which overcomes the drawbacks of prior emissions reducing methods, strategies and systems.
It is a first object of the invention to provide a method of operating a hybrid electric vehicle to reduce emissions which overcomes at least some of the previously delineated drawbacks of prior emissions reducing methods and strategies.
It is a second object of the invention to provide a method of operating a hybrid electric vehicle which utilizes the vehicle""s motor/generator to selectively increase and reduce the power output of the internal combustion engine to reduce emissions.
It is a third object of the invention to provide a method of operating a hybrid electric vehicle to reduce emissions which utilizes the vehicle""s electric motor/generator to reduce the light-off time of the catalytic converter.
It is a fourth object of the invention to provide a method of operating a hybrid electric vehicle which reduces emissions by utilizing the vehicle""s motor to reduce the load changes experienced by the vehicle""s internal combustion engine during transient events.
According to a first aspect of the present invention, a method is provided for reducing emissions of a hybrid electric vehicle of the type having a drive train, an electric machine which selectively provides torque to the drive train, an engine which selectively provides torque to the drive train and which operates at a certain power output, and a catalytic converter which receives exhaust gas from the engine and which operates at a certain efficiency based upon temperature. The method includes the steps of detecting a cold-start condition; and selectively providing a negative torque to the drive train during the cold-start condition by use of the electric machine, the negative torque being effective to increase the power output of the engine and the temperature of the catalytic converter, thereby increasing the efficiency of the catalytic converter and reducing emissions.
According to a second aspect of the present invention, a method is provided for reducing emissions of a hybrid electric vehicle of the type having a drive train, an electric machine which selectively provides torque to the drive train, an engine which selectively provides torque to the drive train and which operates at a certain power output, and a catalytic converter which receives exhaust gas from the engine. The method includes the steps of detecting a transient event; and selectively providing a positive torque to the drive train by use of the electric machine, the positive torque being effective to supplement the torque provided by the engine and lower the certain power output, thereby lowering exhaust mass flow through the catalytic converter and reducing emissions.
These and other features, aspects, and advantages of the invention will become apparent by reading the following specification and by reference to the following drawings.